In devices for measuring very low currents, conventional printed circuit board (PCB) designs absorbed unacceptable numbers of electrons or other charge carriers at the low current nodes of the devices. This absorption is referred to as dielectric absorption.
Electric fields (E-fields) are responsible for leakage displacement currents. Leakage displacement currents follow the path of E-fields through the PCB material. As an E-field persists through the material of the PCB, the material relaxes and the polar molecules of the material align with the E-field. When the E-field is removed, the polar molecules of the material of the PCB return to a randomized state. The alignment and return to randomization, however, can take an extended period of time. It is desirable to prevent the E-field from penetrating the PCB material involved with very low current measurements.
Conventional methods reduce the current leakage by replacing most of the PCB dielectric material with air, as shown in FIG. 1 and discussed in U.S. Pat. No. 5,490,325. FIG. 1 shows a PCB 100 with apertures 102 and 104 surrounding an island 106. The island is held in place by two stems 108 and 110. A circuit board trace 112 is provided in the middle of the island. Guards 114 may also be provided near the two stems 108 and 110 to provide a sink for charge collected by the walls of the apertures 108 and 110.
Another conventional method to reduce the current leakage is shown in FIG. 2. In this configuration, a PCB 100 is provided with an island 106 surrounded by an aperture 202. The island 106 is connected to the PCB 100 by low-leakage components 204 and 206. A circuit board trace 112 and guards 114 are provided in this configuration as well. The two stems 108 and 110 are removed once the components are in place. In these prior art devices, however, a reduction in current leakage was accomplished by routing out islands. E-field lines still flowed through the PCB material and the PCB material still aligned with the E-field. Even with the island technique, it still would take a significant amount of time for the polar molecules of the PCB material to randomize after voltage is no longer applied.